The invention relates to electrical connectors and, more particularly, to modular electrical connectors that provide signal paths for differential signals between mother boards and daughter boards or other electrical components.
Specialized electrical connectors may be used to connect different components of an electrical system. Typically, such an electrical connector connects a large number of electrical signals between a series of daughter boards to a mother board. The mother and daughter boards are connected at right angles. The electrical connector is typically modular. For example, a flat, planar metallic lead frame contains several signal paths, each of which bends about a right angle within the plane of the metallic lead frame. The signal paths are assembled into an insulated housing that also contains a planar ground plate that provides a ground path and provides isolation between signals. The module is further assembled with other similar modules to form a connector capable of connecting a large number of signals between components in an electrical system.
Typically, the connectors attach to a printed circuit board, e.g., a mother board, daughter board, or back-plane. Conducting traces in the printed circuit board connect to signal pins of the connectors so that signals may be routed between the connectors and through the electrical system. Connectors are also used in other configurations, e.g., for interconnecting printed circuit boards, and for connecting cables to printed circuit boards.
Electronic systems generally have become more functionally complex. By means of an increased number of circuits in the same space, which also operate at increased frequencies. The systems handle more data and require electrical connectors that are electrically capable of carrying these electrical signals. As signal frequencies increase there is a greater possibility of electrical noise being generated by the connector in forms such as reflections, cross-talk and electromagnetic radiation. Therefore, the electrical connectors are designed to control cross-talk between different signal paths, and to control the characteristic impedance of each signal path. In order to reduce signal reflections in a typical module, the characteristic impedance of a signal path is generally determined by the distance between the signal conductor for this path and associated ground conductors, as well as both the cross-sectional dimensions of the signal conductor and the effective dielectric constant of the insulating materials located between these signal and ground conductors.
Cross-talk between distinct signal paths can be controlled by arranging the various signal paths so that they are spaced further from each other and nearer to a shield plate, which is generally the ground plate. Thus, the different signal paths tend to electromagnetically couple more to the ground conductor path, and less with each other. For a given level of cross-talk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors is maintained.
An early use of shielding is shown in Japanese patent disclosure 49-6543 by Fujitsu, Ltd. dated Feb. 15, 1974. U.S. Pat. Nos. 4,632,476 and 4,806,107 (both assigned to ATandT Bell Laboratories) show connector designs in which shields are used between columns of signal contacts. These patents describe connectors in which the shields run parallel to the signal contacts through both the daughter board and the back-plane connectors. U.S. Pat. Nos. 5,429,520, 5,429,521, 5,433,617, and 5,433,618 (all assigned to Framatome Connectors International) show a similar arrangement.
Another modular connector system is shown in U.S. Pat. Nos. 5,066,236 and 5,496,183 (both assigned to AMP, Inc.), which describe electrical modules having a single column of signal contacts, and signal paths arranged in a single plane that parallels the ground plate. In contrast, U.S. Pat. No. 5,795,191, which is incorporated herein by reference, describes an electrical module having electrical signal paths arranged in two parallel planes that each couple to a different ground plate.
It appears that the foregoing electrical connectors are designed primarily with regard to single-ended signals. A single-ended signal is carried on a single signal-conducting path, with the voltage relative to a common ground reference set of conductors being the signal. For this reason, single-ended signal paths are very sensitive to any common-mode noise present on the common reference conductors. We have recognized that this presents a significant limitation on single-ended signal use for systems with growing numbers of higher frequency signal paths.
Further, existing high frequency high density connectors often require patterns and sizes of holes in the attached printed wiring boards (PWB) that limit the width and number of printed circuit signal traces that may be routed through the connector footprint portion of the PWB(s).
We have recognized that, predominantly in a printed circuit backplane, it is highly desirable to have the ability to route on each signal layer multiple traces in various directions between particular patterns, rows, or columns of holes in the connector footprint. We have also recognized that in higher frequency backplane applications, especially for long path lengths, the ability to route wider traces can be used to reduce conductor losses.
We have also recognized that better control of cross-talk can be obtained by designing connectors for differential signals. Differential signals are signals represented by a pair of conducting paths, called a xe2x80x9cdifferential pairxe2x80x9d. The voltage difference between the conductive paths represents the signal.
Differential pairs are known in such applications as telephone wires and on some high speed printed circuit boards. In general, the two conducting paths of a differential pair are arranged to run near each other. If any other source of electrical noise is electromagnetically coupled to the differential pair, the effect on each conducting path of the pair should be similar. Because the signal on the differential pair is treated as the difference between the voltages on the two conducting paths, a common noise voltage that is coupled to both conducting paths in the differential pair does not affect the signal. This renders a differential pair less sensitive to cross-talk noise, as compared with a single-ended signal path. We have invented an electrical connector well suited for carrying differential pairs.
In addition, it is advantageous to have symmetrical, balanced electrical characteristics for the two conductive paths of a differential pair. Because current connectors have signal paths of different lengths (as shown in FIGS. 2 and 3), the electrical delay of each path is not equal, which can degrade the differential signal quality by inducing skew. It would be highly desirable to have a differential connector that has balanced paths.
Further, it would be desirable to have a differential connector module that is compatible with existing modular connector components. It would also be desirable to have a connector with a circuit board hole pattern that supports multiple wide signal traces and improved routability.
One aspect of the invention is an electrical connector module for transferring a plurality of differential signals between electrical components. The module has a plurality of pairs of signal conductors with a first signal path and a second signal path. Each signal path has a contact portion at each end of the signal path, and an interim section extending between the contact portions. For each pair of signal conductors, a first distance between the interim sections is less than a second distance between the pair of signal conductors and any other pair of signal conductors of the plurality.
Another aspect of the invention is an electrical connector module for conducting differential signals between electrical components, the connector module having opposing sides terminating along an edge. The module contains a pair of signal conductors optimized for coupling to the differential signal. The conductors are disposed in the module. Each one of the conductors has a contact portion that is laterally spaced along the edge of the module. Surface portions of the pair of conductors pass from the contact portions through the module in a substantially overlaying relationship along a direction extending through the sides of the module.
Each embodiment of the invention may contain one or more of the following advantages. The impedance of each differential signal path is matched. Each signal path of the pair of differential signal conductors is of equal electrical length. The pairs of differential signal paths can be space closer together. The spacing of each pair of differential signal conductors from other pairs reduces cross-talk within the connector. The pair of differential signal conductors can couple to the ground plate to allow other pairs of differential signal conductors to be placed closer to the signal paths-without inducing cross-talk. A portion of the shield plate can extend between each of the pairs of differential signal conductors. Noise within each pair of differential signal conductors is reduced. The routing of signal traces is efficient. The grounding contact portions can extend between the contact portions of the signal conductors and allow the signal traces to extend in a direct path through a routing channel. The routing channel can be wide and straight.
Still another aspect of the invention is a printed circuit board for receiving contact tail portions of a connector. The printed circuit board includes a surface exposing pairs of apertures configured to receive the contact tail portions of the connector, and at least one ground plane layer. For each pair of apertures, an area surrounding the pair of apertures free of the ground plane layer is provided.